TBS: change default disk usage threshold to 0.8

[carsonip]

Motivation/summary

Change TBS default usage threshold to 0.8 for a lower risk. Change log to be slightly more readable.

With this change, the log looks like

setting disk usage threshold to 80% of total disk space of 912.7gb

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

see #15467 (comment)

Related issues

Changing the default set in #15467
Part of #14933

[mergify]

This pull request does not have a backport label. Could you fix it @carsonip? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-9./d is the label to automatically backport to the 9./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.